Fiber-reinforced composite materials containing an epoxy resin composition serving as a matrix resin have been widely used for aircrafts, automobiles, and industrial applications due to their superior mechanical properties and the like. Especially, as for structural and interior materials for aircrafts, from the perspective of achieving lighter weight, there are more cases where fiber-reinforced composite materials are used as face sheets of honeycomb panels.
Conventionally, a honeycomb panel is produced in so-called co-cure molding in which a film-type adhesive agent is placed on both sides of a honeycomb core, a prepreg (uncured fiber-reinforced composite material) serving as a face sheet is laminated on the both sides of the honeycomb core, and curing of a resin constituting the prepreg and adhesion between the face sheet and the honeycomb core are performed simultaneously. In recent years, aiming to obtain a lighter honeycomb panel and to reduce cost for molding, a so-called self-adhesion technique for directly bonding a honeycomb core and a prepreg has been demanded.
For causing a prepreg to exhibit self-adhesiveness, it is important to form a fillet by wetting a bonding surface between a honeycomb core and the prepreg with a resin of the prepreg in heat curing, so-called fillet forming, and to improve the shape and strength of the fillet. The fillet is formed by the resin in a state of hanging or climbing from the prepreg in a thickness direction of the honeycomb core along a wall of the honeycomb, and has a shape deeply associated with the viscosity of the resin. In addition, the strength of the fillet is affected by the toughness of a matrix resin constituting the prepreg.
Patent Document 1 discloses, regarding a thermosetting resin composition comprising a thermosetting resin and a thermoplastic resin, that the toughness of the thermosetting resin is improved by achieving a co-continuous phase morphology thereof after being cured. However, this thermosetting resin composition, although the toughness of the thermosetting resin is improved to some extent, has not necessarily been sufficient for improving toughness necessary for improving the strength of a fillet formed when a honeycomb and a prepreg are directly bonded.
Patent Document 1: Japanese patent application Kokai publication No. Hei. 2-305860